This invention pertains to inline color laser imaging devices, as well as electrophotographic development processes, and in particular to methods and apparatus for reducing calibration time in an inline color imaging device.
Color printing by an electrophotographic printer is achieved by scanning a digitized image onto a photoconductor. Typically, the scanning is performed with diodes which pulse a beam of energy onto the photoconductor. The diodes can be for example laser diodes or light emitting diodes (LEDs). The photoconductor typically comprises a drum or a belt coated with a photoconductive material capable of retaining localized electrical charges. Each localized area capable of receiving a charge corresponds to a pixel. Each pixel is charged to a base electrical charge, and then is either exposed or not exposed by the laser, as dictated by the digital data used to pulse the laser. Exposing a pixel corresponds to electrically altering (typically discharging) the localized area from the base electrical charge to a different electrical charge. One charge will attract toner, and the other charge will not. In this manner, toner is selectively transferred to the photoconductor. In most electrophotographic printing processes, the exposed (electrically discharged) pixels attract toner onto the photoconductor. This process is known as discharge area development (DAD). However, in some electrophotographic printing processes the toner is attracted to the un-discharged (i.e., charged) area on the photoconductor. This latter type of electrophotographic printing is known as charge-area-development (CAD). For purposes of discussion, it will be assumed that DAD is used, although the present invention is not limited to DAD.
Once the photoconductor has had the desired toner transferred to it, the toner is then transferred to the finished product medium. This transfer can direct, or it can be indirect using an intermediate transfer device. The finished product medium typically comprises a sheet of paper, but can also comprise a transparency. After the toner is transferred to the finished product medium, it is processed to fix the toner to the medium. This last step is normally accomplished by thermally heating the toner to fuse it to the medium, or applying pressure to the toner on the medium. Any residual toner on the photoconductor and/or the intermediate transfer device is removed by a cleaning station, which can comprise either or both mechanical and electrical means for removing the residual toner.
There are a variety of known methods for selectively attracting toner to a photoconductor. Generally, each toner has a known electrical potential affinity. Selected areas of the photoconductor are exposed from a base potential to the potential for the selected toner, and then the photoconductor is exposed to the toner so that the toner is attracted to the selectively exposed areas. This latter step is known as developing the photoconductor. In some processes, after the photoconductor is developed by a first toner, the photoconductor is then recharged to the base potential and subsequently exposed and developed by a second toner. In other processes, the photoconductor is not recharged to the base potential after being exposed and developed by a selected toner. In yet another process, the photoconductor is exposed and developed by a plurality of toners, then recharged, and then exposed and developed by another toner. In certain processes, individual photoconductors are individually developed with a dedicated color, and then the toner is transferred from the various photoconductors to a transfer medium which then transfers the toner to the finished product medium. The selection of the charge-expose-develop process depends on a number of variables, such as the type of toner used and the ultimate quality of the image desired.
Image data for an electrophotographic printer (which will also be known herein as a xe2x80x9claser printerxe2x80x9d), including color laser printers, is digital data which is stored in computer memory. The data is stored in a matrix or xe2x80x9crasterxe2x80x9d which identifies the location and color of each pixel which comprises the overall image. The raster image data can be obtained by scanning an original analog document and digitizing the image into raster data, or by reading an already digitized image file. The former method is more common to photocopiers, while the latter method is more common to printing computer files using a printer. Accordingly, the technology to which the invention described below is applicable to either photocopiers or printers. Recent technology has removed this distinction, such that a single printing apparatus can be used either as a copier, a printer for computer files, or a facsimile machine. In any event, the image to be printed onto tangible media is stored as a digital image file. The digital image data is then used to pulse the beam of a laser in the manner described above so that the image can be reproduced by the electrophotographic printing apparatus. Accordingly, the expression xe2x80x9cprinterxe2x80x9d should not be considered as limiting to a device for printing a file from a computer, but should also include a photocopier capable of printing a digitized image, regardless of the source of the image.
The raster image data file is essentially organized into a two dimensional matrix. The image is digitized into a number of lines. Each line comprises a number of discrete dots or pixels across the line. Each pixel is assigned a binary value relating information pertaining to its color and potentially other attributes, such as density. The combination of lines and pixels makes up the resultant image. The digital image is stored in computer readable memory as a raster image. That is, the image is cataloged by line, and each line is cataloged by each pixel in the line. A computer processor reads the raster image data line by line, and actuates the laser to selectively expose a pixel based on the presence or absence of coloration, and the degree of coloration for the pixel.
The method of transferring the digital raster data to the photoconductor via a laser, lasers or LEDs is known as the image scanning process or the scanning process. The scanning process is performed by a scanning portion or scanning section of the electrophotographic printer. The process of attracting toner to the photoconductor is known as the developing process. The developing process is accomplished by the developer section of the printer. Image quality is dependent on both of these processes. Image quality is thus dependent on both the scanning section of the printer, which transfers the raster data image to the photoconductor, as well as the developer section of the printer, which manages the transfer of the toner to the photoconductor.
A typical in-line color laser printer utilizes a plurality of (typically 4) laser scanners to generate a latent electrostatic image for each color plane to be printed. This allows for four colors to be imaged on a transfer medium and then transferred to the finished product medium. The four color planes typically printed, and which are generally considered as necessary to generate a relatively complete palate of colors, are yellow, magenta, cyan and black. That is, the color printer is typically provided with toners in each of these four colors. These colors will be known herein as the xe2x80x9cprimary colorsxe2x80x9d. Some printers have the capability of printing one base color on top of another on the same pixel, so as to generate a fuller palate of finished colors. One method to accomplish this is to provide four photoconductors, one for each primary color, used in conjunction with an intermediate transfer belt. This configuration is described more fully below with respect to the prior art apparatus shown in FIG. 1.
In the scanning process, a laser is scanned from one edge of the photoconductor to the opposing edge and is selectively actuated or not actuated on a pixel-by-pixel basis to scan a line of the image onto the photoconductor. The photoconductor advances and the next line of the image is scanned by the laser onto the photoconductor. The side-to-side scanning of each laser is traditionally accomplished using a dedicated multi-faceted rotating polygonal mirror which causes the laser beam to be scanned across the photoconductor at the unique relative lineal position from a first edge to a second edge of the photoconductor. As the mirror rotates to an edge of the polygon between facets, the laser is essentially reset to the first edge of the photoconductor to begin scanning a new line onto the advancing photoconductor.
For color printing, it is important to assure the registration of the different colors. That is, each laser should be aligned with respect to the other lasers such that a given pixel in the raster image is associated with a single common point on the transfer medium, regardless of which laser is used to identify the point. A registration which is xe2x80x9coffxe2x80x9d will result in a blurry image, or an image with colors not representative of the raster image. Registration is thus dependent on aligning all of the lasers in a laser printer with respect to one another, a process known as calibration. Each laser and its associated components (i.e., rotating mirror, optical elements, and deflector mirror) is typically mounted in a precision housing to keep the components in relative fixed position with respect to one another. Assuring registration of the lasers requires aligning the four housings within the printer itself. As environmental conditions within the printer change (e.g., temperature), this alignment can change. Mechanical vibration or shock to the printer can also allows the lasers to become misaligned.
Since only partial calibration of the laser beams with respect to one another can be achieve by aligning the housings which contain the scanning assemblies, in-line color printers are typically also provided with a calibration system to allow for factory and ex-factory calibration of the lasers. One component of the calibration system is color plane sensors to sense color plane alignment. Sensors are provided to detect shifts in color planes in both the side-to-side scanning direction (the xe2x80x9cscanxe2x80x9d direction), as well as in the direction of advance of the transfer medium (i.e., the xe2x80x9cprocessxe2x80x9d direction). The sensors can provide a feedback to the scanning system and corrections can be made to reposition the laser beam using various known electrical and mechanical methods.
In addition to color plane alignment, color density is another criteria to which a color imaging device can be calibrated. That is, to faithfully reproduce an original image, the density of the toners as applied to the photoconductor should be applied in a manner such that the brightness, contrast and gamma (color density) of the colors appear the same in the generated image as they appear in the original image. This can be achieved by one or more of the processes of varying the quantity of toner applied to the photoconductors, by varying the combination of toners, or by varying the pixel spacing of the toner or toners as they are applied to the photoconductor.
In addition to color registration and density, another characteristic that can be important to achieving a high quality resultant image is faithful reproduction of the spectrum of the colors which are in the original image. That is, a color characterized by a given wavelength in the original image should preferably be reproduced to essentially the same wavelength in the finished image. Since the toners themselves can affect the spectral aspects of the finished product, it is desirable to provide a mechanism to compensate for toner variations, as well as other variables within the imaging apparatus which can affect spectral aspects of the final image. Such a mechanism can attempt to correct spectral variances by varying the mix of toners applied to a pixel, as well as the quantity of each toner applied.
To determine when a color density or spectrum is accurately imaged on the transfer medium, the calibration system can be further provided with color density sensors and color spectrum sensors which can detect the characteristics of a color (e.g., brightness, contract, gamma, and spectral characteristics). Preferably, the calibration sensors are configured to sense the colors deposited on the intermediate transfer medium by the photoconductors (in an inline color printer), rather than on the final printed image, since the medium on which the toner is ultimately deposited can have attributes affecting the color properties.
In order to generate an image on the transfer medium which has known properties against which a standard can be compared, most color imaging devices are provided with a calibration image of known qualities. The qualities can comprise images of known geometric patters (e.g., circles, squares, etc.), color types (e.g., a color of a known wavelength, a color of a known intensity, etc.), and color proximities (e.g., blue adjacent to red). The calibration image is typically stored in computer readable memory which is preferably resident within the imaging device itself. When calibration is to be performed, either automatically or as directed by a user, the color imaging device generates the calibration image on the intermediate transfer medium to produce a calibration product. The calibration product is then moved past the calibration sensors to detect the applied colors. The sensors send output signals to a processing unit (preferably resident within the imaging apparatus). The output signals from the calibration sensors can be stored temporarily in computer resident memory. The output signals are then compared to the calibration image on a selected pixel-by-pixel basis to determine if the calibration product varies from the calibration image, and if so, by how much.
The processor can be further provided with an algorithm to determine what correction(s), if any, need to be made to bring the calibration product into conformance with the calibration image. The corrections can include for example adjusting timing of the activation of the lasers to affect the relative positions of the application of one toner with respect to another, adjusting the intensity of the lasers to affect the amount of toner deposited on a photoconductor (and hence onto the transfer medium), adjusting the positions of energy beams from the lasers directed onto the photoconductors, and adjusting the rotational speeds of the individual photoconductors. Other adjustments are also possible. After adjustments are made, it can be preferable to generate another calibration product to determine whether or not the adjustments have brought the various components of the imaging apparatus into conformance with the calibration image.
The prior art methods for generating a calibration product are configured such that as the transfer medium moves past the various optical photoconductors (xe2x80x9cOPCxe2x80x9d) in a direction from the first OPC to the last OPC, a first toner is applied to the transfer medium by a first OPC most distal from the calibration sensor, a second toner is then applied to the transfer medium by a second OPC, and so on until the last toner is applied to the transfer medium. The calibration product is then passed by the calibration sensors. It is thus obvious that the overall calibration time includes the time between the initial application of the first toner to the transfer medium, and the time the first-applied toner passes by the first calibration sensor. Since calibration can be automatically invoked each time the imaging apparatus is put into service, it is apparent that the calibration time can affect the availability of the imaging apparatus until the calibration process is completed. It is therefore desirable to find a way to reduce the calibration time for inline color imaging devices to thus improve the availability of the device to users.
FIG. 1A depicts a schematic side elevation diagram of a four laser color electrophotographic imaging apparatus (xe2x80x9cprinterxe2x80x9d) 10 which can be used to implement the prior art calibration method. The printer 10 comprises a scanning section 12 and a photoconductor section 14, also known as the developing section. The scanning section, or xe2x80x9cexposure sectionxe2x80x9d 12 comprises a plurality of lasers, typically one laser for each photoconductor station. The coherent beams of energy from each laser are directed to a dedicated photoconductor in the developing section via a rotating reflective mirror and other optical components. The photoconductor section 14 shown in FIG. 1A comprises four optical photoconductors (xe2x80x9cOPCsxe2x80x9d) 16, 18, 20 and 22. In the configuration shown, each OPC is provided rotating drum which subsequently transfers toner from the OPC to the transfer medium, belt 24. In an alternate embodiment, the toner can be applied directly to the transfer medium 24 without the use of the four individual OPCs. The belt 24 rotates in the direction xe2x80x9cAxe2x80x9d to move toner deposited on the belt past a calibration sensor 28. The belt is supported by rollers 26. In a printing mode, a sheet of finished product medium xe2x80x9cMxe2x80x9d, for example a sheet of paper, is passed in close proximity to the belt 24. A toner transfer module 42, typically an electrostatic charge unit, transfers toner from the belt 24 to the medium xe2x80x9cMxe2x80x9d. The toner is then fused to the medium at the fusing station 44. Any residual toner remaining on the belt 24 is removed by the cleaning station 40 before the belt returns to the OPC section 14.
The printer 10 further comprises a processing unit 30 which controls the discharge of the lasers in the exposure section 12 to selectively expose each OPC in the exposure section 14. The selective exposure is generated in response to a digital file version of an image to be produced collectively by the four colors. The processing unit 30 is in electrical signal communication with a computer readable memory 32. The computer readable memory 30 is provided with a digital file version 34 of the calibration image, and is further provided with a calibration product generation algorithm 46. The calibration product generation algorithm comprises a series of steps which can be executed by the processor 30 to direct the scanning section 12 to generate the calibration image product on the transfer belt 24.
FIGS. 2A through 2D depict the calibration product generation method of the prior art. The printing apparatus 10 of FIG. 1A is shown in each figure in simplified form. In the prior art method, the toner most removed from the calibration sensor 28 is applied to the transfer belt 24 first, as shown in FIG. 2A. This toner is depicted as T4, applied by OPC16. Following the application of toner T4 to the transfer belt, the toner second-most removed from the calibration sensor 28 is applied. This is depicted in FIG. 2B wherein toner T3 is applied by OPC 18 after toner T4 has passed the OPC. The next prior art calibration step is shown in FIG. 2C, wherein toner T2 is applied following the application of toner T3. The final step is shown in FIG. 2D, wherein the toner T1 nearest the calibration sensor 28 is applied by OPC 22 after the other toners T4, T3 and T2 have passed by the OPC.
As is apparent from FIG. 2C, at this stage in the calibration product generation process the calibration sensor 28 can begin detecting the calibration image product. It is also apparent from FIG. 2C that the time between the initiation of the calibration product generation and the sensing of the first portion of the calibration product is the time it takes the belt 24 to travel from point P1 at OPC 16 to the calibration sensor 28. During this interval the printer is unavailable to print jobs as requested by a user. It is thus desirable to find a way to reduce the time between initiation of calibration product image generation and first sensing of the calibration product.
The invention includes methods and apparatus for reducing the time required to generate a calibration image product on a transfer medium in an inline imaging device having a plurality of toner stations by simultaneously applying at least two toners to the transfer medium.
In a first embodiment of the present invention, a method for generating a calibration product for an image producing device having a plurality of toner stations and a transfer medium is disclosed. Each toner station is configured to deposit an associated toner onto the transfer medium to thereby generate the calibration product. The method includes the step of simultaneously depositing a first toner from a first toner station and a second toner from a second toner station onto the transfer medium while moving the transfer medium from a first position relative to the plurality of toner stations to a second position. The method can further include the step of simultaneously depositing a third toner from a third toner station onto the transfer medium while moving the transfer medium from the first position relative to the plurality of toner stations to the second position.
In a second embodiment of the present invention, a first toner from a first toner station is deposited onto the transfer medium while moving the transfer medium from a first position relative to the plurality of toner stations to a second position. A second toner from a second toner station is then deposited onto the transfer medium while continuing to deposit the first toner onto the transfer medium while moving the transfer medium from the second position to a third position. The method can further include the step of selecting the first toner to be applied to the transfer medium as a function of a length of a portion of the calibration product to be defined by the first toner and the distance between the first toner station and the second toner station.
An apparatus in accordance with the present invention includes a scanning section, a plurality of toner stations, a transfer medium, and a calibration image product generator. Each toner station is configured to deposit a dedicated toner onto the transfer medium. The transfer medium is configured to move past the toner stations in serial order, and the scanning section is configured to cause selective deposition of toner associated with a toner station onto the transfer medium. The calibration image product generator is configured to cause the scanning section and the plurality of toner stations to produce a calibration image product on the transfer medium as the transfer medium moves past the toner stations. The calibration image product generator is further configured to cause at least two toner stations to simultaneously deposit toner onto the transfer medium.
The apparatus can further include a processor in signal communication with the scanning section, and a computer readable memory accessible by the processor. The calibration image product generator comprises a series of computer executable steps stored in the computer readable memory, the computer executable steps being configured to generate the calibration image product.